added rounded corners & created file for rounded corners

chrome/android/java/res/layout/tabular_context_menu_page.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2017 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file. -->
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/content_layer"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:minWidth="280dp"
     android:orientation="vertical">
     <LinearLayout
         android:id="@+id/context_header_layout"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:layout_marginTop="20dp"
-        android:layout_marginStart="20dp"
-        android:layout_marginEnd="20dp"
-        android:layout_marginBottom="10dp"
+        android:paddingTop="20dp"
+        android:paddingStart="20dp"
+        android:paddingEnd="20dp"
+        android:paddingBottom="10dp"
         android:layout_gravity="center"
         android:orientation="vertical"
         tools:ignore="UseCompoundDrawables">
         <!-- Cannot use a compound drawable since it does not allow a change in maxHeight and
              background to the image. -->
         <ImageView
             android:id="@+id/context_header_image"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_gravity="center"
@@ skipping 23 matching lines @@
         android:background="#1e000000"/>
     <ListView
         android:id="@+id/selectable_items"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:paddingTop="8dp"
         android:paddingBottom="8dp"
         android:dividerHeight="0dp"
         android:divider="@null"/>
 </LinearLayout>